Homologous Series (SQA National 5 Chemistry): Flashcards

True.

Saturated hydrocarbons have all carbon atoms joined by single C-C bonds only, with no double bonds present.

The two homologous series of saturated hydrocarbons are:

1. Alkanes

2. Cycloalkanes

When bromine solution is added to an alkene, the orange-brown colour disappears because the alkene undergoes an addition reaction with the bromine.

The alkene's C=C double bond breaks and bromine atoms add across it in an addition reaction. This uses up the bromine, so the orange-brown colour disappears.

An unsaturated hydrocarbon is one that contains at least one C=C double bond between carbon atoms.

False.

Alkanes are saturated and cannot undergo addition reactions, so they do not react with bromine solution and the orange-brown colour remains unchanged.

The correct observation is that the orange-brown colour disappears and the solution turns colourless. Saying "it goes clear" alone would not gain the mark.

Add a few drops of the hydrocarbon to a test tube, add a few drops of orange-brown bromine solution, stopper the tube and gently shake. An unsaturated compound turns the solution colourless; a saturated compound shows no visible change.

A molecular formula shows the total number of each type of atom in a molecule, but gives no information about how the atoms are connected.

True.

A full structural formula is the most detailed representation, displaying every individual atom and each bond between them.

The shortened structural formula of propane is CH₃CH₂CH₃. Each group shows one carbon and its attached hydrogen atoms.

The condensed structural formula for ethanol is written as C₂H₅OH to show the hydroxyl functional group separately.

Write each carbon atom and its attached hydrogen atoms as a group in order along the chain. For butane this gives CH₃CH₂CH₂CH₃, reading from one end of the chain to the other.

A shortened structural formula is a line of text that groups the hydrogen atoms with each carbon in order along the chain, summarising the structure without drawing every bond.

True.

Carbon always forms four bonds. Checking that each carbon has exactly four bonds is the key rule for verifying a full structural formula.

The shortened structural formula of ethanoic acid is CH₃COOH. The carboxyl group (-COOH) is written at the end to highlight the functional group.

A molecular formula shows only the total number of each type of atom, whereas a shortened structural formula groups atoms by carbon to give information about how the molecule is connected.

Isomers are molecules that have the same molecular formula but different structural formulae, meaning their atoms are connected in a different order.

False.

Although isomers share the same molecular formula, their different structural arrangements mean they typically have different physical properties, such as different boiling points.

Structural isomers are compounds that have the same molecular formula but different structural arrangements of atoms.

Yes. Isomers share the same molecular formula but have different structures, which means they can have different physical properties including boiling points.

Isomers are compounds with the same molecular formula but a different structural arrangement of atoms.

Not necessarily. Isomers have the same molecular formula but can have different physical properties such as boiling point and melting point.

True.

Both propene and cyclopropane have the molecular formula C₃H₆, but they belong to different homologous series and have different structural formulae.

Pentane has three structural isomers: pentane (straight chain), 2-methylbutane (one branch) and 2,2-dimethylpropane (two branches on the central carbon).

Carboxylic acids do not have positional isomers because the carboxyl group (-COOH) is always located at the end of the carbon chain (carbon 1) and cannot be moved to another position.

Alkanes are a homologous series of saturated hydrocarbons containing only carbon and hydrogen atoms, with all carbon-carbon bonds being single bonds (C-C).

True.

When an alkane burns completely in oxygen, the only products are carbon dioxide and water. This reaction is highly exothermic.

Using the general formula C_nH_2n+2 with n = 8 gives C₈H₁₈. The number of hydrogen atoms is (2 × 8) + 2 = 18.

In the name of a branched alkane, branches are named using the same prefix as the chain length but with the ending changed from -ane to -yl.

Alkanes do not contain any -OH or other groups that can form bonds with water molecules, so they do not dissolve in water.

1. 1 carbon: meth-

2. 2 carbons: eth-

3. 3 carbons: prop-

4. 4 carbons: but-

False.

Alkanes are saturated hydrocarbons because all carbon-carbon bonds are single bonds. Unsaturated compounds contain at least one C=C double bond.

The name is 2-methylbutane. The main chain has 4 carbons (butane) and the 1-carbon branch (methyl) is on carbon 2.

Using the general formula C_nH_2n+2 with n = 7 gives C₇H₁₆. The number of hydrogen atoms is (2 × 7) + 2 = 16.

When naming a branched alkane, you number the carbon atoms in the main chain so that the branch has the lowest possible number.

A cycloalkane is a saturated hydrocarbon in which the carbon atoms are joined by single bonds in a closed ring structure.

False.

Cycloalkanes are saturated — all carbon-carbon bonds are single bonds (C-C). Unsaturated compounds contain at least one C=C double bond.

The general formula is C_nH_2n.

Cyclopropane and propene are isomers of each other because they share the same molecular formula C₃H₆.

They are effective at dissolving other substances, making them useful for dissolving substances that are insoluble in water.

The cyclo- prefix indicates that the carbon atoms in the molecule are joined in a closed ring structure.

False.

A minimum of three carbon atoms is needed to form a ring structure, so the smallest cycloalkane is cyclopropane.

The molecular formula of cyclohexane is C₆H₁₂, using the general formula C_nH_2n with n = 6.

An alkene is an unsaturated hydrocarbon that contains at least one carbon-carbon double bond (C=C) as its functional group.

True.

The C=C double bond is the functional group of alkenes. It is the site of most of their chemical reactions and makes alkenes more reactive than alkanes.

The general formula is C_nH_2n.

Alkenes are used as monomers to make many common plastics by addition polymerisation.

Ethene is the first member because at least two carbon atoms are needed to form the C=C double bond, which is the defining functional group of an alkene.

The -ene ending indicates that the molecule contains a carbon-carbon double bond (C=C) and belongs to the alkene homologous series.

True.

Carbon atoms are numbered from the end that gives the double bond the lowest possible number. The number refers to the first carbon that the double bond is attached to.

The molecular formula of hex-2-ene is C₆H₁₂, using the general formula C_nH_2n with n = 6.

Alkenes are more reactive than alkanes because the C=C double bond is a reactive functional group that can undergo addition reactions, whereas alkanes have only C-C single bonds.

An addition reaction is a reaction in which the C=C double bond of an alkene breaks open and the atoms of another molecule are added across it, forming a single saturated product.

The three main types are:

1. Hydrogenation (addition of hydrogen)

2. Halogenation (addition of a halogen)

3. Hydration (addition of water/steam)

Adding hydrogen to an alkene is called hydrogenation and requires a nickel catalyst and heat.

True.

Hydrogen adds across the C=C double bond, converting it to a C-C single bond and producing a saturated alkane.

The orange-brown bromine solution is decolourised because bromine adds across the C=C double bond of the alkene in a halogenation reaction, forming a colourless dihaloalkane.

Hydration is the addition of water (steam) across the C=C double bond of an alkene to form an alcohol.

Hydration requires high temperature, high pressure and an acid catalyst such as phosphoric acid.

In an addition reaction of alkenes, the C=C double bond becomes a C-C single bond and one product is formed.

Ethene + water (steam) → ethanol